ABSTRACT Discover and functionally characterize full-penetrance causes of nephrosis/FSGS. Chronic kidney diseases (CKD) take one of the highest tolls on human health, and their prevalence continuously rises. Steroid-resistant nephrotic syndrome (SRNS) is the 2nd most frequent cause of CKD before 25 yrs. By focal segmental glomerulosclerosis (FSGS) it inevitably leads to CKD with a 33% recurrence risk in a renal transplant. The pathogenesis of SRNS is unknown and no curative treatment is available. For SRNS, the primary causes (etiology) and disease mechanisms (pathogenesis) have been a conundrum for decades. However, identification of full-penetrance single-gene causes of NS (e.g. podocin) has implicated the renal glomerular podocyte at the center of the pathogenesis. Within the 2 previous R01 funding periods we: 1) Identified by whole exome sequencing 34 of the 50 currently known single-gene causes of NS and functionally characterized the related disease mechanisms; 2) Discovered that the encoded proteins cluster in protein complexes thereby defining novel disease pathways for SRNS (e.g., RhoA/Rac1/Cdc42 signaling); 3) Delineated genotype-phenotype correlations with actionable implications for personalized disease management; 4) Modeled the related disease mechanisms in the `podocyte migration assay', zebrafish & mouse models; 5) Revealed `personalized treatment' options for specific patients (e.g. CoQ10 in COQ6 or ADCK4 mutations); 6) Demonstrated in a world-wide cohort that ~30% of SRNS (<25 yrs) is caused by single-gene mutations, thereby permitting genetic mechanistic studies and personalized medicine for patients with SRNS ; 7) Discovered the first genetic causes of steroid-dependent NS (6 genes), converging on RhoA regulation. These genetic discoveries made the study of SRNS accessible to genetic approaches of `precision medicine', enabling genetic diagnostics, the study of `personalized' disease mechanisms, and treatment approaches. We, therefore will pursue the following Specific Aims: SA1. Discover the missing single-gene causes of SRNS by WES in ~1,000 SRNS families. SA2. Functionally characterize newly identified single-gene causes of SRNS/SSNS to delineate the pathogenesis and study `personalized' genotype-phenotype and genotype-treatment correlations. SA3. Perform small molecule screens in CRISPR k.o. models of novel SRNS genes identified, using established `podocyte migration assay' and zebrafish models, to discover the first drugs for SRNS. SA4. Study the 6 novel single-gene causes that we discovered in steroid-dependent NS to converge on RhoA regulation delineate mechanisms of steroid and other direct drug effects on podocytes.